linux-kernel - Re: [PATCH] mm: Prevent derefencing NULL ptr in pfn_section

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <bf0d9278-2b57-079e-cca5-21163e187ea3@quicinc.com>
Date: Mon, 1 Jul 2024 19:20:21 +0530
From: Charan Teja Kalla <quic_charante@...cinc.com>
To: Waiman Long <longman@...hat.com>,
        Andrew Morton
	<akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Prevent derefencing NULL ptr in pfn_section_valid()


Hi Waiman,

On 6/26/2024 5:46 AM, Waiman Long wrote:
> Commit 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing
> memory_section->usage") changed pfn_section_valid() to add a READ_ONCE()
> call around "ms->usage" to fix a race with section_deactivate() where
> ms->usage can be cleared.  The READ_ONCE() call, by itself, is not enough
> to prevent NULL pointer dereference.  We need to check its value before
> dereferencing it.

I am unable to see a scenario where ms->usage will be NULL when
pfn_section_valid() is called:

1) In pfn_valid, valid_section() check ensures that pfn_section_valid()
is not called as the section is marked as invalid.

2) In pfn_to_online_page, online_section() check ensures that
pfn_section_valid() is not called.

and in the update path, we do:
     kfree_rcu(ms->usage, rcu);
     WRITE_ONCE(ms->usage, NULL);

Could you help me in understanding about what I am missing here, please?

> 
> Fixes: 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing memory_section->usage")
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  include/linux/mmzone.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 8f9c9590a42c..b1dcf6ddb406 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1980,8 +1980,9 @@ static inline int subsection_map_index(unsigned long pfn)
>  static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>  {
>  	int idx = subsection_map_index(pfn);
> +	struct mem_section_usage *usage = READ_ONCE(ms->usage);
>  
> -	return test_bit(idx, READ_ONCE(ms->usage)->subsection_map);
> +	return usage ? test_bit(idx, usage->subsection_map) : 0;
>  }
>  #else
>  static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)